Hi 6BG6GA,
If you are willing to help Marian by doing this, it would make his life much easier I think. Not only that, but there are many of us in the field who would benefit as well.

Now, I'm not sure how much I can add by running point, but I can until it's clear that you ought to be leading the way! Your comment concerning the protection circuit might be really accurate since Marian says he has issues with a resistor feeding that circuit.

In the mean time, I'm off to eat and I'll review your diagrams (thank you for these!).

I'm sorry to be the bearer of bad news, but that's how it is, thing is the owner decided that he could not wait anymore ( i swear it's true ), and took the amp back, he sayd that he wants to try with another technician and if he cannot fix it either than he would do as i sugested, he will let me built him an entirely new amplifier, a mosfet one, something like AV400 or similar.

Now i find myself in an awkward position, and i feel verry bad cus you all tryed to help me and it was all for nothing, and i trully apologise for that but it is out of my control, i thank all of you so much for youre imput, even if i did not fix that amp it is an experiance that has much to offer.

I hope you will believe me and most of all will not hate me, and i wish all of you all the best.

Unfortunately there are some problems with amplifiers that cannot be fixed in 10 min or less. Your friend should have let you work on this a few days. My suggestion is that you contact your friend and buy a few more days with the ailing 7300A. I can and will obtain ALL the information needed to enact a complete repair of this animal. Building and or buying something else will not be the solution in my opinion. Its far more cost effective to repair this old work horse and have something capable of taking some abuse. If you lived in this country I would ship you a working EV2400, or Altec 9444B/SA but shipping to your country would be excessive. Lets get it back so we can fix it.

The 7300A,7300,9444b, 9444B/SA, EV2400, and EV3600 are stout amplifiers. They aren't nearly as stout as the Altec 9440 (my favorite) but they are surely better than most out there in a commercial and or industrial setting or in a situation where they will receive a lot of abuse.

The 7300 has separate +15 and -15 volt regulators and doesn't suffer the same faults as the the 9444B/SA and the amplifiers in that family. The protection circuit in the 9444 series must be perfect in order for the supply voltage to be correct to the Op Amp. Usually the caps in the protection circuit start to get leaky and then you will obtain a lop sided supply to the OP Amp and as you know it just doesn't work correctly then.

Doesn't really sound like the action of a friend to me, were it me (and that's just me ) I would say this guy is on his own. I'd steer well clear of further involvement - someone this impatient isn't likely to be all that understanding if there are further problems with the amplifier down the road. Maybe this was your lucky break..

You should not feel bad that you asked for help here, these are just the breaks sometimes..

Sorry to resurect this topik but the owner of the amp came back with it asking me to try again and fix it, so i have made further verifications and came up with this:

1.I have verified the power transistors again, took them out of the board and after measuring them again and comparing the results with measurements made on the devices of the working module i have found them to be similar, almost identical at both diode junctions and Hfe.

2.I have measured the voltages directly on the filter caps, taking as refference point the common point of the caps that way i knew the rails had to be the same, on the working channel i found almost 1V difference between the rails, and it stayed stable, but on the defective one initialy the values were the same as the other one but, after a few seconds the pozitive rail started to drop ( again the refference point was not the floating ground, but the common point between the filter caps ), the voltage on the pozitive rail kept dropping at a slow but steady pase. then i have taken out of the circuit all the power transistors, nothing changed, the voltages remained the same and the pozitive line kept on dropping down many volts, then i conclude that the power transistors cannot be at fault, so the problem would seam to be the common point between the filter caps ( that floating ground ) goes twards the possitive rail, the cause remains unknown ( i've tryed the caps out of the circuit and that way the rails are the same and stable ).

3.I checked after that the drivers, but they seam to be ok, and after removing theyr power i have found that they do not influence the main rails.

4.Now here comes something i did not observed the other time, i have measured the voltage on the out pin of the OP amp, and found +5,6Vdc ( the working module does not have this issue ) versus the ground, then knowing that a Dc current on the exit of the op amp could influence the Offset voltage at the loudspeaker terminals i have removed the resistance that makes the conection between the op amp and the power amp, that way the voltage on the possitive rail went up at +71Vdc and this time the negative line dropped down at -42Vdc, but the protection stil is on.

Now i found myself at loss again, so i come to you for help, i'm sorry i do not have a cleaner schematics of the amp, but i present you some pics i took of the amp, and awayt any cind of imput from you, meanwhile i will try to draw a new schematics of the amp in eagle so it could be better understud.